Description of the Prior Art
The present invention is particularly suitable for use as patient control in hospitals, nursing homes and other establishments where ambulatory or other personnel may be free to move about but where some means is needed to detect their movement beyond certain pre-selected monitoring stations while allowing other personnel to pass freely through these monitoring stations.
Various electronic type systems have been proposed in the past for monitoring personnel movement. One such system is described in U.S. Pat. No. 3,115,622 to D. L. Jaffe. According to this patent, personnel to be monitored are provided with a miniature portable radio transmitter which may be worn around the neck and this radio transmitter emits signals which are detected through a receiving antenna which surrounds an area in which the personnel are normally stationed. The Jaffe system requires the provision of a battery or other electrical power source on the transmitters. In addition, it is necessary for the portable transmitters to emit signals continually and consequently that a rather substantial power supply is required for these transmitters. This, of course, makes the transmitters quite large and uncomfortable to wear.
U.S. Pat. No. 3,500,373 to Arthur J. Minasy relates to an improvement over the Jaffe system in that the Minasy system incorporates a purely passive responder circuit. The Minasy patent describes a theft detection system wherein a passive resonant circuit is embedded in a label or wafer attached to an article of merchandise to be protected. When the article passes a check station it encounters an electromagnetic field which is continually emitted at the station. Means are also provided at the station to monitor for predetermined electromagnetic responses produced at the station antenna when a resonant circuit passes by the antenna. The wafers used in the Minasy system are flat and have a relatively large expanse (e.g. 6.5 .times. 9.2 cm) to provide sufficient area for reacting effectively with the signals emitted by the station antenna. However, a flat expensive element of this size may be uncomfortable or bothersome for a person to wear for extended periods of time.